The invention relates to exploring a subsurface earth formation for valuable underground resources, such as oil and gas, and relates specifically to producing a tangible record, such as a map or a trace, showing the dip and azimuth of bedding surfaces of the earth formation which intersect a borehole which is in that formation. Dip is the angle between the vertical and a vector normal to the bedding surface, and azimuth is the angle between a given direction, say true North, and the projection of the same vector on a horizontal plane.
In well-logging, measurements of various earth formation characteristics are taken by an investigating tool which is passed through a borehole at the end of a supporting cable extending from the surface of the earth. Measurements, which are called samples, are taken at specified intervals by one or more logging devices carried by the tool as the tool is drawn up. Typically, these measurements are of earth formation characteristics such as resistivity, conductance and the like. A set of measurements of the same characteristic is called a log. Such logs are used in various ways to find oil or gas bearing strata. The samples may be recorded directly, or transmitted via a transmission system, into electronic signal processing devices. The recorded or transmitted samples may be processed as digital signals by specially set up general purpose digital computers or, in the alternative, wholly or partly by special purpose signal processing circuitry.
Because of the desirability of finding out more accurately how and at what depth the borehole is intersected by a bedding surface, sophisticated prior art techniques have been used in an effort to find the difference in depth between the places where a feature of a bedding surface intersects the respective paths along the borehole of respective logging devices carried by the tool. These prior art approaches include the use of a process which in effect looks for the degree of similarity of relevant parts of two logs. One example of the techniques used in the prior art is disclosed in application Ser. No. 537,998 filed on Dec. 30, 1974 in the name of C. Clavier, A. Dumestre and V. Hepp and assigned to the assignee of this invention.
Some knowledge about a bedding surface can be found by looking for the degree of similarity between relevant portions of the logs produced by two logging devices moving along a respective first and second path along the borehole. Additional knowledge about the same bedding surface may be found by looking for the degree of similarity between the logs from different pairs of paths along the same borehole. Prior art techniques which employ a process involving the similarity of relevant parts of different pairs of logs in an effort to find the difference in borehole depth between the points of intersection of a feature of a bedding surface with the paths of logging devices moved along the borehole, are proposed in U.S. Pat. Nos. 2,920,306; 2,297,656 and 2,928,071, all issued to Feagin et al. As proposed in these patents, relevant portions of a pair of logs are depth-shifted with respect to each other until they appear to be most similar. Various functions are used to find the relative depth shift of places at which the features of the bedding plane intersect the borehole. These patents are believed to suggest that the relative depth shift between two log portions is independent of other logs and must be determined independently. In U.S. Pat. No. 3,725,981 issued to Sasseen et al, and in an article, "An Electronic Analog Cross Correlator for Dip Logs" by J. H. Sasseen, in the IRE Transactions on Electronic Computers, September 1975, page 182, there are proposals for looking for such similarity of log portions when employing dipmeter tools which produce logs along more than two paths in a borehole. A triple cross-correlator is discussed, and a technique for obtaining the best fit of the relevant log portions is also discussed. A procedure for finding the similarity between the relevant portions of a triple of logs is suggested, although in certain cases more than one answer is obtained for local peaks of apparent similarity. This occasional ambiguity presents a problem in that there is a correct end result only if the arbitrary selection of an initial relative depth shift between two log portions, to serve as an entry point to the procedure, fortuitously turns out to have been correct. No prior art technique is known which can produce a tangible record of the attitude of a bedding surface with the accuracy and efficiency of the invention described below.